Fossil fuel combustion is used in industrial processes for many different purposes. Unfortunately, fossil fuel combustion produces several contaminants, which have been found to be detrimental to the environment. In particular, sulfur and nitrogen oxide compounds are the major components of “acid rain”
In recognition of the harm caused by SOx and NOx compounds, different combustion gas cleaning processes have been developed to remove these components of combustion flue gases prior to release of the flue gases into the atmosphere, especially since burning fossil fuel releases many millions of tons of SO2 every year.
According to the Oslo Protocol, lime/limestone wet scrubbing (LWS) makes up 85% of all flue gas desulfurization (FGD) installations. In 1995 the FGD sector accounted for 15% of all lime sales in the US, mostly for LWS. In the LWS process the acidic, gaseous SO2 in the flue gas is adsorbed into a re-circulated water based slurry of pretreated limestone with gypsum, and is subsequently neutralized by a reaction with CaCO3. The products of the reaction are further reacted with air to produce predominantly calcium sulfate di-hydrate. The SO2 removal capacity is dependent upon the inlet SO2 content of the gas, the relative flow rate of the slurry and the pH of the slurry.
Another important desulfurization process is SO2 removal by neutralization with an NaOH solution. Caustic soda is a strong base and is highly soluble in an aqueous solution, thus, a high pH and high absorption capacity can be achieved. However, the cost of NaOH is about ten times higher than that of lime/lime stone.
U.S. Pat. No. 6,447,437 describes a method for the reduction of air pollution caused by SO2 by absorbing it with ammonia to form ammonium sulfite/sulfate followed by a stage of an anion-exchange reaction in which the sulfite/sulfate is exchanged by nitrate or phosphate anions to form a sulfur compound and a highly valued fertilizer; ammonium nitrate or ammonium phosphates.
Fertilizers are used in volumes of millions of tons per year. They are used all over the globe and throughout most of the year. Some of said fertilizers are salts produced by direct neutralization of an acid and a base. The main examples are ammonium nitrates, which are usually formed by the direct reaction of ammonia and nitric acid, and ammonium phosphates, which are usually formed in a reaction between phosphoric acid and ammonia. This direct reaction produces neutralization energy. The number of applications for said neutralization energy is limited. Therefore, choosing a fertilizer as a high value by-product of SO2 removal has a great advantage.
As mentioned above U.S. Pat. No. 6,447,437 suggests producing the fertilizer through indirect neutralization of ammonia with said nitric acid or phosphoric acid by combining it with the absorption of SO2 from flue gas. However, using ammonia as the neutralizing base in those systems is practically difficult. The flue gases are emitted at high temperatures of about 200-400° C. whereas the ammonia b.p. is −33° C., while ammonia salt tends to decompose at high temperatures. In addition, the ammonia has an unpleasant odor, the gas irritates all parts of the respiratory system and is extremely irritating to the eyes.
A major disadvantage of this process is that a combined production of two main products that are totally different is usually undesirable, especially in industries involved with the combustion of oil and coal for electric power stations. These stations are not characterized by high technology for fine chemical products. Fertilizer production requires certain specifications with which electric power stations are not associated.
Therefore, there is still a great need for a desulphurization process that is characterized by an efficient absorption of SO2 from flue gases, and ease of use even in high temperatures. In addition, this process should enable the recycling or selling of all by-products and thus avoid discharging pollution into the environment.
It will thus be realized that the present invention serves to combine two processes: (I) the process of reducing air pollution caused by SO2 by using a common desulphurization process, i.e., such as the simple lime/limestone wet scrubbing or wet scrubbing with NaOH; and (II) the process of the combined production, from the by-product resulting from said first process, of (a) a fertilizer containing ammonium nitrate or ammonium phosphate; and (b) a sulfur oxide containing product.
A further purpose of the present invention is to utilize the driving force that is associated with the production of the fertilizer, said fertilizer being available in large amounts, in many locations and throughout most of the year, for the production of sulfur compounds from sulfur oxide containing gases.